Not Applicable.
Not Applicable.
The present invention relates to improved membranes for separation of gas or liquid or vapor mixtures. In particular, this invention relates to improved polymeric composite membranes and a process for preparation of the membranes.
Gas or vapor or liquid separation via membranes is an established commercial technology with many applications and continues to find acceptance in new applications. Among the applications are: (a) separation of hydrogen from nitrogen, methane, or carbon monoxide in applications such as recovery of ammonia purge gas, oil refining, and synthesis gas manufacture; (b) separation of carbon dioxide and hydrogen sulfide from methane in the upgrading of natural gas; (c) separation of oxygen from nitrogen in the production of nitrogen enriched air or oxygen enriched air; (d) separation of water vapor from compressed air or natural gas to obtain a dry gas; (e) separation of volatile organic compounds (VOC) from air or nitrogen, (f) recovery of fuels from air or nitrogen in transloading operations; (g) recovery of fluorinated hydrocarbons from nitrogen in the manufacture of semiconductors; (h) pervaporative separation of water from liquid alcohol mixtures; and (i) pervaporative separation of trace organic compounds from aqueous streams. In each of these applications, membranes compete with other separation technologies, e.g., absorption in solvents, adsorption in molecular sieves or other adsorbents, distillation or refrigeration. The choice of separation technology employed depends upon a variety of factors, including capital cost of the separation equipment, energy cost per unit volume of gas produced, reliability, maintenance costs, ease and flexibility of operation, and size and weight of the separation equipment.
Useful membranes have a thin dense layer which provides the selectivity or separation characteristics and a porous substructure which provides mechanical support. Membranes used in gas or vapor or liquid separations may function based on one of three general transport mechanisms: (1) solution diffusion, (2) Knudsen diffusion, or (3) selective sieving by molecular size. Polymeric membranes used in commercial product offerings for gas or vapor or pervaporative liquid separation, function almost exclusively based on solution diffusion. Permeation via solution diffusion involves dissolution of a permeating species at one interface of the membrane, diffusion through the polymer membrane, and desorption at the opposite membrane interface. The driving force for permeation through the membrane is the partial pressure difference between the two sides. In pervaporative separation or pervaporation, a liquid mixture contacts one side of the membrane and the permeate is removed as a vapor from the other side.
The primary requirements of a commercial membrane are a high permeation coefficient (also referred to in membrane literature as xe2x80x9cp/lxe2x80x9d which is defined as the flux of the component per unit of partial pressure difference) for the faster permeating species, high selectivity (i.e., ratio of the permeation coefficient for the faster permeating species to the permeation coefficient for the slower permeating species), stability under the operating feed pressure and temperature, and tolerance to feed stream components and contaminants. Of these, the first two requirements appear to be diametrically opposed to each other because of the inverse relationship of the permeability of a polymer and the selectivity of that polymer for a given set of permeating species. For instance, usually, the more permeable the polymer, the lower its selectivity. This problem can be solved by applying a polymer of adequate selectivity as a coating or laminate to a support to prepare a thin composite membrane which simultaneously realizes a high permeation coefficient and a high selectivity.
Integrally skinned asymmetric membranes represent one class of commercial membranes with thin selective layers. The thin selective layer or skin and the substructure of an integrally skinned, asymmetric membrane are made of the same polymer in a single process. Their inherent limitation is that the permeation properties are derived from the support polymer, and there is a limited number of polymers from which integrally skinned, asymmetric membranes can be produced economically.
Composite membranes represent another class of thin polymer membranes. The selective layer and substructure of composite membranes are made of different polymers, usually in two or more separate process steps. Composite membranes are especially attractive when the selective layer polymer is expensive or lacks adequate mechanical properties to be a useful support. In such composite membranes, the porous support provides the mechanical strength while offering low resistance to transport of the gas or vapor. It is necessary that the surface pores of the porous support be sufficiently small that the thin selective layer bridging the mouths of the pores has adequate burst strength. Composite membranes overcome the inherent limitations of integrally skinned asymmetric membranes so that a wide range of materials can be used for the selective layer. Thus the properties of composite membranes can be tailored to particular applications.
Membranes provide an alternative to desiccant and refrigerant dehydrators used for compressed air drying and to glycol absorption or molecular sieve or deliquescent dehydrators used in natural gas drying. In order to be competitive with conventional technology, the membrane needs to possess a high permeation coefficient for water and high selectivity relative to the other components of the gas mixture. In addition, the membrane should be stable in contact with the feed components and contaminants under operating conditions.
Several methods for the preparation of membranes for dehydration have been described in the literature. One method involves coating a porous support with a solution of a polymer in a solvent mixture. The resulting separating layer may contain a single polymer or a blend of several materials, or multiple coatings with dissimilar polymers. Examples are U.S. Pat. Nos. 4,981,498 and 5,067,971 to Bikson et al. which describe a composite membrane for the dehydration of gases prepared by coating a porous support with a thin layer of a sulfonated polysulfone.
Interfacial polymerization has been used to form the selective layer directly on the porous support by reacting two immiscible reagents (e.g., polyamine in water with diacid chloride in an immiscible organic solvent) from which a cross-linked polymer film is formed on or in the support at the interface of the two reagents, as illustrated in U.S. Pat. No. 5,002,590 to Friesen et al.
As described above, many attempts have been made to provide composite membranes with both high permeation rates and high selectivity. The porous supports used in these membranes are desirably porous to provide low resistance to transport of gas or vapor species and still provide adequate mechanical support to a selective layer.
Permeation of species through a selective layer and the surface pores of the support has been described by a mathematical model (see Keller and Stein, J. Mathematical Biosciences, 1, 421-437, 1967). This type of model illustrates that not all the surface of the selective layer permits permeation; the fractional effective area for permeation increases as the surface porosity (void area fraction) of the support increases and the diameter of the surface pores decreases. Hence, high surface porosity values and small surface pore sizes are desirable in a porous support. In addition, smaller pore sizes provide better mechanical support to the thin selective layer thus preventing the rupture of the thin layer under operating conditions of pressure and temperature.
One of the problems in the preparation of composite membranes, especially when coating from a solution of the polymer in a solvent system, involves penetration of the polymer coating into the pores of the support. Penetration, even to a minute extent, can severely reduce the permeation rate through the membrane. This has been demonstrated via mathematical models of permeation through composite structures (see Lopez et al., J. Membrane Science, 27, 301-325, 1986). During coating with polymer solutions, among other factors, the extent of penetration of the coating solution into the support depends upon the molecular weight of the polymer and its concentration in the solution. High molecular weight and high concentration of the polymer in solution are desirable to reduce the extent of penetration; however, these factors also result in thicker coated layers and hence lower permeation rates. Smaller surface pore sizes are desirable to reduce penetration into the porous support. However, typical preparation techniques for porous supports result in lower surface porosity as the surface pore size is decreased; this tends to result in a decrease in the permeation rate through the composite membrane.
In light of the above, there exists a need for further improvement in the preparation of composite membranes particularly suitable for dehydration of gases or liquids. This is especially true for overcoming the problem of excessive penetration by the selective layer into a porous support layer when the support layer high surface porosity. In addition, it is necessary to have a manufacturing process which is simple and low in cost.
The present invention provides a composite membrane for separation of gas or vapor or liquid mixtures. The composite membrane of the present invention comprises a porous support provided with a selective layer comprised of a vinylacetate polymer.
For the purpose of defining terminology used herein, the vinylacetate polymer may be a vinylacetate homopolymer or a vinylacetate copolymer. Homopolymer generally refers to a polymer made from essentially one monomer. Copolymer refers to a polymer made from two or more monomers. Classes of copolymers include random alternating, block, or graft copolymers. Examples of suitable monomers which may be used to make the vinylacetate copolymers of the present invention include the following: vinyls such as vinylacetate, vinyl chloride, vinylidene chloride, alkyl vinyl ether, halogenated alkyl vinyl ether; acrylonitriles such as acrylonitrile methacrylonitrile; alkenes, such as ethylene, propylene, 4-methyl 1-pentene, butadiene, and the like; halogenated alkenes such as, tetrafluoroethylene, hexafluoropropylene, vinylidene fluoride; acrylates such as alkyl acrylates, halogenated alkyl acrylates; methacrylates such as alkyl methacrylates, halogenated alkyl methacrylates; acrylamides; styrene and substituted styrenes such as styrene, methylstyrene, halogenated styrene; and allyl compounds such as allyl acetate, allyl chloride, and allyl bromide.
The composite membrane of the present invention is prepared by coating a porous support with a colloidal dispersion or emulsion or suspension (hereafter referred to as a colloidal dispersion) of a vinylacetate polymer, preferably followed by heat treatment to provide a selective layer. The vinylacetate polymer layer provides a high permeation coefficient and selectivity for certain components versus the other components of a gas or liquid or vapor mixture.
The membranes of the present invention may be flat sheets or hollow fibers or any other suitable membrane configuration.
The present invention also provides a process for preparation of a vinylacetate polymer composite membrane by contacting a surface of a porous support with a colloidal dispersion or emulsion or suspension of a vinylacetate polymer, preferably in an aqueous medium. After contact with the colloidal dispersion, the support preferably is subjected to heat treatment, or other suitable treatment, to cause coalescence of the colloidal particles and formation of a substantially continuous thin layer.
Colloidal dispersions or emulsions or suspensions are generally systems of particles dispersed in a continuous liquid phase and are characterized by slow diffusion and slow sedimentation of the dispersed particles under normal gravity, the dispersed particles generally having a size in the range of about 10 to about 10,000 Angstroms diameter. Lyophobic colloidal dispersions contain particles which are insoluble in the continuous liquid phase and may require the presence of a stabilizing substance for their preparation. The stability (i.e., a long shelf life) of such colloidal dispersions requires that the particles repel each other, for example, by carrying a net electrostatic charge or by being coated with a sufficiently thick layer of large molecules compatible with the liquid phase (J. Th. G. Overbeek, Colloidal Dispersions, Royal Society of Chemistry, 1981). The polymers of the selective layer of the composite membranes of the present invention, are provided by lyophobic polymer colloidal dispersions comprising particles dispersed in a suitable liquid, preferably water.
Polymer colloidal dispersions can be prepared by the following processes: emulsion polymerization, dispersion polymerization, or suspension polymerization. Molecular weight can be controlled by initiator concentration, temperature control, or the addition of chain transfer agents which reduce molecular weight (D. J. Walbridge, Solid/Liquid Dispersions, Academic Press, 1987; J. Langley, Technological Applications of Dispersions, Dekker, 1994). Particle size can be controlled by the presence of and the concentration of an ionic surfactant and/or a non-ionic surfactant. Fine microemulsions can be obtained from the above processes.
Another method of preparing polymer colloidal dispersions involves mixing a dilute solution of the polymer and suitable surface active agents (or surfactants) in a suitable solvent under high shear with suitable non-solvent (for the polymer). A stable colloidal emulsion is formed if the concentrations of the components are properly selected, particularly the concentration of the polymer and the concentrations of the surfactants.
Polymer colloidal dispersions can also be prepared by mechanical means, e.g., crushing, abrading, grinding, centrifugal force in colloid mills, and ultrasonic methods.
The colloidal polymer dispersion may be monodisperse or polydisperse with respect to particle size. The particle diameter distribution should be such that minimal penetration of the colloidal particles into the surface pores of the porous support occurs because too large a degree of penetration will result in a composite membrane which has a low permeation coefficient. However, a small degreed of penetration is preferred to xe2x80x9canchorxe2x80x9d the selective layer coating onto the porous support.
The porous supports preferably are prepared from polymer dopes by thermal or phase inversion processes or by other suitable means. The wall of the porous support may have a microporous structure, or it may have macropores with lower porosity at the inside and/or outside surfaces. It is preferred that the porous supports have a much higher surface porosity than typical integrally skinned asymmetric membranes.
Examples of suitable polymers for the porous support are polysulfones, polyethersulfones, polyimides, polyethermides, polyamides, polyamideimides, polyacrylonitrile, polycarbonate, polyarylate, cellulose acetate, polypropylene, and poly(4-methyl 1-pentene).
The selective layer polymer comprises vinylacetate homopolymer, or copolymers of vinylacetate with one or more monomers. Examples of suitable monomers are:
vinyls: vinyl chloride, vinylidene chloride, alkyl vinyl ether, halogenated alkyl vinyl ether, and the like,
acrylonitriles: acrylonitrile, methacrylonitrile, and the like,
alkenes: hydrocarbons, fluorocarbons, chlorocarbons, or bromocarbons, such as ethylene, propylene, 4-methyl 1-pentene, butadiene and the like.
halogenated alkenes: tetrafluoroethylene, hexafluoropropylene, vinylidene fluoride and the like,
acrylates: alkyl acrylates, halogenated alkyl acrylates, and the like,
methacrylates: alkyl methacrylates, halogenated alkyl methacrylates, and the like,
acrylamides and the like,
styrene and substituted styrenes: styrene, methylstyrene, halogenated styrene, and the like,
allyl compounds: allyl acetate, allyl chloride, allyl bromide, and the like.
Particular examples of polymers and copolymers of interest in the present invention include polyvinylacetate homopolymer, vinylacetate-ethylene copolymer, vinylacetate-acrylic copolymer, or vinylacetate-acrylonitrile copolymer.
The polymer colloidal dispersions are deposited on the surface of the porous support by contact with a dispersion of selected vinylacetate polymer particles in a suitable liquid followed by draining the excess dispersion. Hollow fiber porous supports may be coated on either the inside or outside surface utilizing the above process. A pressure may be applied to the dispersion to assist deposition of the solid particles and to compact the deposited layer. Upon evaporation of the continuous phase liquid (e.g., water) the colloidal particles come into close contact, and deform if the temperature is above the minimum film formation temperature. If the temperature is sufficiently high, further gradual coalescence occurs as the polymer in the particles fuses to form a continuous selective layer, in a process which is also called xe2x80x9cannealingxe2x80x9d. Cosolvents, if present during the drying and layer annealing process, assist formation of a continuous layer.
The colloidal particles may be xe2x80x9ccross-linkablexe2x80x9d. For example, they may have reactive groups which undergo cross-linking via chemical reaction forming covalent linkages, or hydrogen-bonding, or ionic or dipolar interactions, during the process of selective layer formation resulting in formation of a tough and more durable selective layer with improved stability in contact with feed components and contaminants under the operating conditions. Examples of cross-linking groups include hydroxyl, carboxyl, acrylic, epoxy or other groups that undergo cross-linking reactions with or without the presence of an initiator or catalyst during or after the drying process. If the polymer does not contain reactive groups, cross-linking agents, such as monomers and initiators, may be added to the colloidal dispersion to allow cross-linking reactions to occur during or after formation of the selective layer.
The addition of surfactants in the polymer colloidal dispersion may help in the wetting and spreading processes that occur during deposition of the colloidal particles on the surface of the porous support. However, the presence of certain surfactants or excessive amounts of surfactant in the selective layer coating may cause a reduction in the permeation coefficient or selectivity of the composite membrane. Hence, the selection of a surfactant and its concentration should be optimized to produce a dispersion which allows adequate deposition of the colloidal particles and also results in a selective layer coating with desirably high permeation coefficient and selectivity.
The polymer colloidal particles are deposited on the surface of the porous support utilizing the action of attractive forces such as Van der Waals forces, hydrogen bonding forces, and other forces active in chemisorption of molecules onto a surface. These forces also assist the adherence of the final selective layer coating to the support. If necessary, the surface of the porous support is pretreated by any suitable method to increase its surface energy to enhance deposition of the dispersion and provide good adherence of the final selective layer coating.
After coating with the polymer colloidal dispersion and drying or annealing the coating, the membrane preferably is xe2x80x9crepair coatedxe2x80x9d with a solution of silicone in a volatile solvent (e.g., a hydrocarbon, such as isopentane) using the process of U. S. Pat. No. 4,203,463 to Henis et al. Optionally, the silicone solution is applied to the coated surface with a vacuum applied to the opposite side of the membrane.
The coatings of this invention applied to the porous supports via deposition of polymer colloidal dispersion and selective layer formation are expected to provide selective layers which are non-porous, and hence, the composite membranes thus prepared are expected to be useful in separation of gas or vapor or liquid mixtures. The membranes thus formed have advantages over integrally skinned asymmetric membranes in gas and vapor or liquid separation applications, the membranes of the invention exhibiting (a) higher permeation coefficient for the faster permeating species, (b) lower permeation coefficient for the slower permeating species, (c) higher selectivity for the faster species relative to the slower species, (d) reduced degradation of separation characteristics under operating conditions, and (e) improved stability in contact with the components and contaminants of the feed stream.
The thickness of the selective .layer coating deposited on the surface of the porous support is influenced by many factors, among which are (a) concentration of polymer particles in the dispersion, (b) concentration and type of surface active agents present, (c) pH of the dispersion, (d) temperature, and (e) net pressure applied across the porous support during contact with the dispersion.
The composite membrane preferably has a thin selective layer coating on the surface of the porous support in order to allow a high permeation coefficient. However, the selective layer coating should be sufficiently thick to withstand the pressure difference across the membrane during operation of the separation process.
The preferred thickness of the selective layer coating is the lowest value that provides stable permeation coefficients at the operating conditions. At a minimum, the coating thickness should be sufficient to avoid rupture or collapse of the membrane at the operating pressure and temperature.
The following examples are by way of illustration only and are not intended to limit the scope of the invention.